(1) Field of the Invention
This invention relates to a solid electrolyte capacitor, and more particularly to the capacitor having a electroconductive polymer as a solid electrolyte, and its manufacturing method.
(2) Description of the Related Art
The solid electrolyte capacitor having an anode body of a valve metal such as tantalum and aluminum has been widely used. It is possible for such as electrolyte capacitor to enlarge the area of its dielectric layer by preparing the dielectric using such an anode body as a sintered body or an etched foil, and thus to have a relatively large capacity in spite of a small size. It is disadvantageous, however, in that it gives a high impedance when exposed to a radio frequency because it uses manganese dioxide or ethyleneglycol as an electrolyte.
Recently, electric appliances become compact, and come to have multiple functions. In such a tendency, digitalization is introduced widely and demand for capacitors with an excellent performance in high frequency range has been intensified. To meet such demand new capacitors have been developed that incorporate, as a solid electrolyte, an electroconductive polymer having a conductivity several hundreds times as high as that of conventional ones. As one of such electroconductive polymer may be mentioned a compound which is produced after a dopant has been added to a complex, five-membered ring compound such as polypyrrol. The resulting electrolyte capacitor has an excellent frequency characteristics that has never been achieved by previous solid electrolyte capacitors. One of such capacitors is disclosed in Japanese Examined Patent Publication No. 4-56445.
Generally, the solid electrolyte capacitor which incorporates such an electroconductive polymer is superior not only in ESR (equivalent series resistance) characteristics and capacitance property, but in reliability to conventional capacitors. Take, for example, a conventional electrolyte capacitor which incorporates, as an electrolyte, manganese dioxide which has been produced through thermal cracking. The new solid electrolyte dispenses with the use of thermal treatment for its formation, and hence its oxide film is free from damages due to heating.
Applying an electroconductive polymer layer on an oxide film can be achieved by two methods: one is chemically oxidized polymerization and the other is electrolytically oxidized polymerization. Electrolytically oxidized polymerication is achieved by applying firstly a thin coat of an electroconductive polymer obtained through chemically oxidized polymerication, or of manganese dioxide on an oxide film to serve as a precoat, and then by resorting to electrolytically oxidized polymerization, using the precoat as an electroconductive layer. The reason why such precoat is prepared lies in that the oxide film, being an insulating body, can not transfer electric charges. Through above procedure, the electrolytically oxidized polymerization can produce a layer of an electroconductive polymer with a sufficient thickness.
The solid electrolyte capacitor incorporation such electroconductive polymer is advantageous in that its solid electrolyte has a low resistance. Therefore, various modifications have been added to that capacitor to ameliorate other characteristics in addition conductivity, thus to improve its overall utility.
For example, in Japanese Unexamined Patent Publication No. 3-64013 or No. 3-64014 is disclosed a solid electrolyte capacitor which has a surfactant inserted between an oxide film acting as a dielectric and an electroconductive polymer acting as a solid electrolyte, to form an electroconductive polymer layer evenly and efficiently on the surface of an oxide film, an object which is shared by the present inventors. To put it in more detail, the surfactant facilitates adherence of a polypyrrol polymer to the surface of the oxide film, thereby ensuring firm adherence of the pyrrol oligomers close to the surface of the oxide film thereto, and thus efficient and even formation of the electroconductive layer on the surface of the oxide film. This constitution will prevent deterioration of the capacitance and maximum durable voltage, and improve tan .delta..
Or in Japanese Unexamined Patent Publication No. 2-71021 or No. 7-73924 is disclosed a solid electrolyte capacitor which has, like the foregoing, a silan coupling agent, titanium coupling agent or aluminum coupling agent inserted between a dielectric oxide film and an electroconductive polymer layer. These two inventions intends to provide a solid electrolyte capacitor comparatively free from deterioration or loss of capacity and performance under high temperatures, by taking advantage of the property of those coupling agents which can improve the affinity between the oxide membrane and electroconductive polymer. These inventions thus intends to provide a solid electrolyte capacitor reliable even under a high-temperature environment, thereby to meet one of the demands often directed to this type of capacitors.
As described above, a method whereby, in a solid electrolyte capacitor incorporating an electroconductive polymer as a solid electrolyte, the adhesiveness of an oxide film is improved and an electroconductive polymer layer is efficiently and evenly formed on the surface of the oxide film is disclosed in Japanese Unexamined Patent Publication Nos. 3-64013 and 3-60414. The present inventors made a detailed study on the formation of an electroconductive polymer close to the surface of an oxide membrane, and found that the methods disclosed in above publications only improves adherence of oligomers close to the surface of the oxide film to that surface, the method is limited in applicability because it is ineffective to oligomers which form apart from the surface of the oxide film, and the method allows compounds too immature to be electroconductive to exist on the surface of the oxide film.
As shown in schematically in FIG. 4A, on the surface of an oxide film or an insulator exist a positive zeta potential 1. When an electroconductive polymer layer 7 is formed on the oxide film 2, polymerization reaction proceeds through positive radicals 4, and hence a repellent force is generated, as shown in FIG. 4B, between the polymerizing compound and the oxide film. Accordingly, polymerization proceeds at sites in the solution apart from the oxide film, effective polymerization does not take place close to the surface of the oxide film, and there are places (immature polymer clusters 9) where the compound does not undergo polymerization sufficiently to form an electroconductive body.
When an immature polymer cluster develops on the surface of the dielectric oxide film, it may cause three problems described below.
The first problem is inability to obtain a specified capacity. An oxide film upon which an electroconductive layer is not formed can not work as a dielectric, and thus is not possessed of a capacity. Accordingly, to obtain a specified amount of capacity, it becomes necessary to use a larger amount of tantalum powder than is normal, which will lead to an augmented production cost and enlarged volume of the resulting capacitor.
The second problem is bigger changes in capacity when the capacitor is exposed to an atmosphere, or especially to a humid atmosphere. As shown in FIG. 5, to the surface of an oxide film upon which immature polymer cluster 9 is present (or no electroconductive layer is formed), moisture 10 in the atmosphere reversibly attaches and detaches according to changes in temperature and humidity. When moisture adheres to the oxide film, the capacitance increases because the moisture acts as an electrode. On the contrary, when moisture detaches from the oxide film, the capacitance decreases. This is the reason why the capacitance changes greatly in such capacitor. The capacitor with such characteristics is unsuitable to be applied for a circuit which requires a high stability in capacitance such as a time-constant circuit.
The third problem is lowered reliability. Generally speaking, an oxide film upon which no electroconductive layer is formed is sensitive to changes in impurities ions contained in moisture in the atmosphere for the same reason as discussed above. To put it specifically, when moisture invades the oxide film where no electroconductive layer is formed, anions such as chloride contained in the moisture migrate into the substance of the oxide film which may lead to disorders such as lowered insulation. This in turn results in lowered reliability of the capacitor, and further in lowered reliability of the circuit which incorporates such capacitor.